The present invention relates to a hydraulic binder and, more particularly, to a hydraulic binder utilizing a finely ground bulky latently hydraulic blast furnace slag, a sulfate promoter and activating additives. The term xe2x80x9cfinely dividedxe2x80x9d as used herein with reference to the slag refers to a fineness in excess of 5,000 cm2/g Blaine.
Blast furnace slags are a by-product of iron ore refining. In the blast furnace, a liquid slag is formed which floats on the molten iron and is comprised of clay, silica and lime containing components which separate from the iron ore and of limestone and of impurities in the coke at temperatures of about 1900xc2x0. This is referred to as blast furnace slag and is quenched at about 14000xc2x0 C. with water. In this rapid cooling, a highly vitreous granulate is formed which has latently hydraulic binding capabilities. The glassy blast furnace slag cannot be used however as a stand alone binder. Its hardening energy can be activated by the addition of promoters. The activation is basically effected in two ways: by the addition of hydrated lime or cement (alkali activation) and by the addition of calcium sulfate (sulfatic activation). However, not every slag can be activated similarly.
The latent hydraulic binding force of the slag depends upon the composition thereof. Thus, Dr. Fritz Keil in xe2x80x9cCement, Manufacture and Characteristicsxe2x80x9d, Springer-Verlag, Berlin, 1971, has described at page 116 the quality of the slag as depending upon the ratio (CaO+CaS+0.5MgO+Al2O3)/(SiO2+MnO), referred to as the F value. If the F value lies above 1.9, the slag is considered to be very good from a hydraulic binding point of view. Below 1.5, it is considered only moderately hydraulic.
A further criterion of slag activity is the SiO2 content. Slags with less than 32% SiO2 in general are considered highly hydraulic and slags with more than 37% SiO2 are considered as less hydraulic.
With respect to the alkali activation, according to EN (European Standard) 197-1 for cement production, the slag quality must conform to the following requirement: the ratio (CaO+MgO)/SiO2 must be greater than 1.0.
For the sulfatic activation the literature with respect to slag indicates that it should have an Al2O3 content of at least 13% and the ratio (CaO+MgO+Al2O3)/SiO2 should be above 1.6.
Numerous Patents deal with a CaO content in excess of 40% and an Al2O3 content above 14% (see for example the Canadian Patent 1,131,664).
In the Canadian Patent 1,131,664, a binder is described that has the following composition:
80-85% Granulated blast-furnace Slag (with 40-50% CaO, 14-20% Al2O3, 30-35% SiO2, 5-8% MgO).
13-17% CaSO4 (calculated as the anhydrite)
1.5-2.5% Portland Cement
0.1-0.5% of an organic carboxylic acid or a salt thereof
0.03-0.6 methyl cellulose, sodium stearate or sodium laurylbenzoylsulfonate and
0.6-2% sodium sulfate.
High hydraulic slags are used in accordance with those teachings. To improve the slag for this purpose, Al2O3 and CaO can be added to the slag and the mixture then heated.
Based upon the aforedescribed criteria, however, most of the blast furnace slags have been found to be only moderately hydraulic. Such low-value blast furnace slags can be used as additives in the cement industry to a maximum of 65% in cements. Exceeding this proportion rapidly lowers the strength of the cement product. The concrete industry can also use the lower value blast furnace slag as aggregates or additives to a limited extent only.
It is therefore an object of the present invention to provide a binder of the type described which can use a low value slag but nevertheless obtain a high value product.
Still another object of the invention is to produce a binder which is free from drawbacks of earlier systems.
These objects and others which will become apparent hereinafter are attained, in accordance with the invention with a hydraulic binder comprised of a latently hydraulic bulky blast furnace slag, finely ground to in excess of 5,000 cm2/g Blaine, containing a sulfate promoter and activating additives, and wherein the latently hydraulic bulky blast furnace slag has substantially the following analysis:
Glassiness:  greater than 93%,
SiO2: 34 to 40% by weight,
CaO: 34 to 37% by weight,
Al2O3:  greater than 9% by weight, and
(CaO+MgO)/(Al2O3+SiO2) : 0.88-0.98.
It has been found, quite surprisingly, that a slag of this composition or fulfilling these requirements yields good products even when the slag would be the usual criteria described above, be considered just a moderately hydraulic substance.
In other words, following the requirements of the invention, slags with less than 13% by weight Al2O3 can be used and can have F values below 1.5 and a ratio (CaO+MgO+Al2O3)/SiO2 below 1.6.
A single slag alone seldom can satisfy all of these requirements according to the invention. Since, however, there are large numbers of low value slags, it is often possible to form a mixture of at least two blast furnace slags so that the mixture provides all of these requirements. This has the further advantage that the degree of activation is much higher when the above mentioned prerequisites are obtained by a mixture of slags from different sources.
The activation of the slag is effected by the addition, mixing or milling with the slag of natural calcium sulfate (natural gypsum), flue gas desulfurization gypsum (or its anhydrite) in an amount of 12 to 20% by weight, Portland cement in an amount of 0.5 to 5% by weight and/or other CaO carriers like hot dust, burnt lime or Ca(OH)2 in an amount of 0.5 to 3%, alkali sulfates or alkali carbonates in amounts up to 3% by weight and/or calcium salts of carboxylic acids in amounts of 0.5 to 3%. The use of hot dust (hot meal) obtained from flue or kiln gases, for example, has been found to be advantageous and accelerates the hydration of the slag and increases the compressive strength. The hot meal can be a raw meal which has been deacidified to 70 to 90% and of the type which is collected in the lowermost cyclone stage of the heat exchanger furnace at 800 to 900xc2x0 C. in the production of Portland cement clinker. The meal can be recovered by a pass from the furnace system and contains principally CaO.
Prisms product from the binder of the invention show higher resistance to sulfate attack and have strengths in the strength classes of the Austrian standard B3310 (xc3x96NORM) and the European standard (EN) 197-1 and are comparable with conventional Portland cement or better.